Sodium bicarbonate has traditionally been used as a leavening agent in the bakery industry. However concerns about the health implications of high levels of sodium in the diet have led to moves to develop potassium bicarbonate as an alternative. Normal, unmilled, commercial potassium bicarbonate has a mean particle size (expressed as D50, i.e. the aperture size of a sieve that would retain 50% of the weight of a sample) of about 500μ. This is too coarse for use in dough, giving rise to localised discolouration in the final baked product, known as spotting.
Spotting is a particular problem with products prepared from dough, due to the relatively low mobility of the latter, compared to batter. The former may be defined as a paste, in which the weight of solid exceeds that of liquid, whereas the latter is a fluid in which the weight of liquid exceeds that of solid. Visible spotting in dough products occurs whenever the bicarbonate contains a substantial proportion of particles greater than about 200μ. However we have found that even particles too small to cause visible spotting can give rise to localised concentrations of alkalinity in dough, which adversely affect the storage properties of the end product. To avoid these problems we believe that the bicarbonate should preferably have a very fine particle size, ideally substantially all less than 250μ.
A major problem with finely milled potassium bicarbonate is storage stability. Unlike sodium bicarbonate which is relatively stable at normal temperatures, potassium bicarbonate is hygroscopic, which gives rise to caking on storage. The smaller the particle size the worse the caking, and very finely milled products may become unusable due to caking after only a few hours exposure to atmospheric moisture. The problem is particularly severe in palletised products, parts of which are subject substantial compressive forces. Moreover very finely milled products cause problems with dust.
A further problem is the chemical instability of the product when mixed with an acidulant, e.g. when incorporated into a baking powder blend, or self raising flour. This results in premature release of carbon dioxide on standing at ambient temperatures. This problem is particularly severe in gluten-free products for people with special dietary requirements. We believe that the carbohydrates present in wheat flour tend to inhibit the decomposition of the bicarbonate.
Currently two methods are used commercially to alleviate the first of these problems. According to one of these, which was described in U.S. Pat. No. 5,552,084, potassium bicarbonate for baking is mixed with magnesium oxide. Magnesium oxide reacts with the bicarbonate to form magnesium carbonate on the surface of the particles, which keeps the bicarbonate particles separate. However this approach has several disadvantages. It does not fully overcome all the aforesaid problems. In particular it does not overcome stability problems in baking powder. Moreover, when the treatment is applied to bicarbonate with a fine enough particle size to prevent spotting, the product has a relatively low bulk density, which increases the cost of storage, transport and packaging.
An alternative approach has been to add a dessicant such as silicon dioxide which inhibits caking and preferentially absorbs moisture. However silicon dioxide does not prevent degradation of the bicarbonate in baking powder, and has an adverse effect on the physical properties of the blend. In order to control caking to an acceptable degree, finely divided potassium bicarbonate requires levels of silicon dioxide in excess of those normally used. When such a product is added to baking powder, the powder tends to flow too freely.
In addition to its use in baking, there are a number of other applications of potassium bicarbonate for which a fine particle size is desirable, e.g. in order to improve dispersibility, and to provide a greater surface area and increased catalytic activity. The present invention is therefore relevant to such applications. They include dry chemical fire extinguishers, fungicides, animal feed supplements, polymerisation and dehydrogenation catalysts, effervescers, pH modifiers for, e.g. food or soil, cigarette filters for removing acidic gases from smoke, and as an accelerator for fast drying cement.
It is known generally that coating with an inert encapsulant may stabilise particles against degradation and caking but this technique has significant drawbacks, which have deterred those skilled in the production of potassium bicarbonate leavening agents from considering it as a viable approach to solving the above problems. In particular, effective protection usually requires a relatively large amount of encapsulant, which increases the particle size. Encapsulation often inhibits the activity of the encapsulated product. In addition it usually requires specialised plant and is often expensive to apply.
The prejudice in the art against coating or encapsulation is reflected in U.S. Pat. No. 5,225,225 (Thomas et al), which describes mixing alkali metal bicarbonates with various additives which are designed to provide nucleating sites for the release of carbon dioxide during baking Thomas states that her mixtures do not involve coating the bicarbonate, and stresses the use of non-shearing methods of mixing that will minimise any tendency to coat the substrate.